Lighting modules constructed with light emitting diodes (LEDs) of different colors do not have a constant cumulative color locus over their operating life, to be precise on account of temperature-dependent changes in brightness and different ageing of the LEDs depending e.g. on the material system used.
In order to keep the cumulative color locus constant and thus in order to compensate for or readjust changes in brightness of individual LEDs, the related art involves detecting light at a specific geometrical position in the optical system, said light containing if possible the light from all light sources of the same color. The detection usually takes place by means of a brightness or color sensor. Only in rare cases is it possible to realize a point-symmetrical LED arrangement with a central sensor, in which arrangement the propagation paths of the light rays from individual LEDs are of the same length and thus lead to comparable signal strengths at the sensor, as a result of which a compensation can be carried out particularly reliably. This method is no longer applicable, however, in the case of lighting modules in which the LEDs are potted in a common diffuser. Moreover, this method is no longer usable in practice for small lighting modules having very short mean free path lengths (MFP).